DNA Replication in A-Level OCR Biology: Key Concepts | A-Level OCR 生物：DNA复制 考点精讲

📚 DNA Replication in A-Level OCR Biology: Key Concepts | A-Level OCR 生物：DNA复制 考点精讲

DNA replication is a fundamental process that ensures each daughter cell receives an identical copy of the genetic material. For OCR A-Level Biology, you must understand the semi-conservative mechanism, the roles of specific enzymes, and the experimental evidence provided by Meselson and Stahl. This guide breaks down every detail, from the replication fork to proofreading, with paired English and Chinese explanations to help you master the topic.

DNA 复制是确保每个子细胞获得相同遗传物质的基本过程。对于 OCR A-Level 生物学，你必须理解半保留机制、特定酶的作用以及 Meselson 和 Stahl 提供的实验证据。本指南将逐一解析从复制叉到校对的每一个细节，并配有中英双语讲解，助你掌握该主题。

1. The Semi-Conservative Model | 半保留复制模型

Watson and Crick proposed that DNA replication follows a semi-conservative model: each new double helix consists of one original (parental) strand and one newly synthesised (daughter) strand. This ensures high fidelity, as the parent strand acts as a template. The alternative hypotheses—conservative replication (the entire parent double helix is conserved) and dispersive replication (parental strands are fragmented and dispersed into new molecules)—were ruled out by experiment.

Watson 和 Crick 提出 DNA 复制遵循半保留模型：每条新的双螺旋由一条原始（母链）和一条新合成（子链）组成。这确保了高保真度，因为母链充当模板。另外两种假说——全保留复制（整个母双螺旋被保留）和分散复制（母链被打断并分散到新分子中）——均被实验排除。

2. Experimental Evidence: Meselson and Stahl | 实验证据：Meselson 和 Stahl 实验

Meselson and Stahl grew E. coli for many generations in a medium containing the heavy nitrogen isotope ¹⁵N, so that all DNA became ‘heavy’. They then transferred the bacteria to a medium with the normal light isotope ¹⁴N and sampled DNA after one and two rounds of replication. Using density-gradient centrifugation, they separated DNA bands by mass.

Meselson 和 Stahl 将大肠杆菌在含重氮同位素 ¹⁵N 的培养基中培养多代，使所有 DNA 变为“重”链。然后将细菌转移到含正常轻同位素 ¹⁴N 的培养基中，在一轮和两轮复制后取样。通过密度梯度离心，按质量分离 DNA 带。

After one generation in ¹⁴N, all DNA formed a single band of intermediate density (hybrid ¹⁵N-¹⁴N). After two generations, they observed two bands: one intermediate and one light (¹⁴N/¹⁴N), in a 1:1 ratio. This pattern exactly matched the semi-conservative prediction. Conservative replication would have produced a heavy and a light band after one generation; dispersive replication would have produced only intermediate bands at all stages.

在 ¹⁴N 中培养一代后，所有 DNA 形成一条中间密度的单一带（杂合 ¹⁵N-¹⁴N）。两代后出现两条带：一条中间带和一条轻带（¹⁴N/¹⁴N），比例为 1:1。此结果完全符合半保留复制的预测。全保留复制一代后应出现重带和轻带；分散复制则各代均只出现中间带。

3. Key Enzymes and Their Roles | 关键酶及其作用

Several enzymes and proteins coordinate DNA replication. The main ones required for OCR include helicase, DNA polymerase, primase, ligase, single-strand binding proteins (SSBs) and topoisomerase (also called DNA gyrase in prokaryotes).

多种酶和蛋白质协同完成 DNA 复制。OCR 要求掌握的主要

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